CD70 is member of the tumor necrosis factor (TNF) family of cell membrane-bound and secreted molecules that are expressed by a variety of normal and malignant cell types. CD70 is a transmembrane type II protein with its carboxyl terminus exposed to the outside of cells and its amino terminus found in the cytosolic side of the plasma membrane (Bowman et al., 1994, J. Immunol. 152:1756-61; Goodwin et al., 1993, Cell 73:447-56). Human CD70 contains a 20 amino acid cytoplasmic domain, an 18 AA transmembrane domain, and a 155 AA extracellular domain with two potential N-linked glycosylation sites (Bowman et al., supra; Goodwin et al., supra). Specific immunoprecipitation of radioisotope-labeled CD70-expressing cells by anti-CD70 antibodies yields polypeptides of 29 and 50 kDa (Goodwin et al., supra; Hintzen et al., 1994, J. Immunol. 152:1762-73). Based on its homology to TNF-alpha and TNF-beta, a trimeric structure is predicted for CD70 (Petsch et al., 1995, Mol. Immunol. 32:761-72).
CD70 has limited expression on normal tissues in humans. This makes CD70 an attractive target for cancer therapies. CD70 expression has been identified, however, on only a small number of cancers, such as renal cell carcinoma, colon cancer, certain types of Non-Hodgkin lymphoma and multiple myeloma. CD70 expression on cancer cells is typically detected using antibodies that bind to native CD70, such as by immunohistochemistry. Detection of CD70 expression on fixed patient samples has proved problematic, due to poor quality antibodies that lack sufficient specificity for CD70. In particular, cross-reactivity and background staining interfere with detection of CD70 in fixed samples. The present invention solves this and other needs.